i. Technical Field
The present invention relates to screening apparatus for separating accepts and rejects of a slurry in preparing pulps for use in a paper making machine.
ii. Prior Art
Screening apparatus are well known in the art. For example, A. J. Haug in U.S. Pat. No. 1,864,818, discloses a pulp screening machine which employs a rotary screening drum for producing a centrifugal effect on the stock in contact with the drum, and maintains the drum submerged in screened stock and applies a substantially uniform hydraulic pressure to the drum at points spaced uniformly thereabout to obtain substantially equal screening operations about the circumference of the screen drum.
In U.S. Pat. No. 1,974,651, A. J. Haug discloses a pulp screening machine which employs a horizontal screening machine which employs a horizontal screening drum in which the stock is moved adjacent to the surface of the drum in a circumferential direction. In this apparatus, the stock is subjected to a motion so as to create temporary localized reversals of the normal flow of stock through the screening surface in order to loosen the tailings which cling to the screening surface.
G. L. Nelson, in U.S. Pat. No. 3,053,391 discloses a vertically disposed screening apparatus in which the incoming stock is fed into the top of the system and flows between a pair of screens having offset foils running therebetween.
A. C. Martin, in U.S. Pat. No. 4,252,641 discloses a horizontal screening apparatus in which the axes of a cylindrical screen plate and a casing surrounding the screen plate are offset to one another to provide an enlargement in the chamber surrounding the screen plate. A partition is provided in the enlarged area and extends from the inlet opening to approximately the rejects outlet and a baffle is provided about the screen helically and substantially coextensively with the partition to provide a passage of decreasing cross-sectional area approaching the rejects outlet to provide an optimum velocity of material which prevents plugging of the passage by larger particles.
D. A. Goddard, in U.S. Pat. No. 4,316,768 discloses a screening apparatus which includes a centrifugal pump which is mounted for rotation in a housing coaxial with a cylindrical screen which extends about a tubular hub portion of the pump. In this screening apparatus, a slurry is delivered to the top of the centrifugal pump and forced outwardly and upwardly by the pump to the cylindrical screen. The rejects are forced into and collected in a rejects zone for later flushing.
Modern paper machines are very sensitive to pressure pulses. Existing screens usually produce pulses which are sufficiently large such that often "barring" occurs on the paper machine. The troublesome pulses occur when a foil or other pulse creating device passes the accepts outlet on a conventional screen. It is therefore desirable to eliminate or at least minimize such pulses.
It is therefore an object of the present invention to eliminate, or at least minimize, pressure pulses in the accepts flow in a screening apparatus.